Just when you thought it was safe to say that a Tully monster is a proto-fish, along comes a group of University of Pennsylvania scientists saying that the enigmatic beast from prehistory wasn’t even a vertebrate.

The mysterious, and extinct Tullimonstrum gregarium was discovered in 1955 by amateur fossil collector Francis Tully, in the Mazon Creek fossil bed in Grundy County, Illinois, which beautifully preserves not only bones but soft tissue in its sedimentary ironstone.

Unmonstrously, the boneless Tully’s Thing was small – 35 centimeters long at most. The weirdness lies in everything else about it.

Its eyes (or at least round organs associated with dark material similar to pigmentation often found in eyes) peered from the end of long stalks coming out of the middle of its body.

Between these “eyes” sprouted an extraordinarily long, flexible proboscis, at the end of which was a small but gaping mouth, with up to eight sharp fangs on each jaw.

The animal had fins next to its tail and maybe others along its body that the fossil record didn’t preserve.

Fossil of a Tully MonsterGhedoghedo, Wikimedia Commons

In 2016, a group of scientists announced they had identified its backbone and classified it as a lamprey-type fish.

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“This animal doesn’t fit easy classification because it’s so weird,” admits Penn paleobiologist Lauren Sallan. “It has these eyes that are on stalks, and it has this pincer at the end of a long proboscis and there’s even disagreement about which way is up. But the last thing that the Tully monster could be is a fish.”

In fact the two papers that seemingly settled the Cambrian-era Tully monster debate don’t even prove the thing was a vertebrate, claim Sallan and colleagues in a paper in Palaeontology. If anything, they think it wasn’t, and that its initial classification as a kind of worm or mollusk was probably right.

Asked by Haaretz whether the thing might be unidentifiable because it's simply gone and there's nothing like it around any more, Sallan explains: "Whether the Tully monster has living relatives depends on what it is. If it is a heteropod gastropod mollusc, a swimming shell-less snail with teeth and eyes, then it is the only fossil species ever found for a widespread living group. That could explain why it has been hard to identify, as soft animals rot before burial and living ones have undergone million years of additional evolution."

Fortunately, she adds, "the Tully Monster is found alongside the actual lamprey and other fishes, which look just like the ones we have now, so a vertebrate assignment is quite easy to rule out!"

The eyes have it

For some, maybe. In 2016, two papers based on studying over 1,000 specimens of Tully reported a band running down the creature’s midline, which they decided was a primitive spine. The papers also claimed to discern internal organ structures, gill sacs and teeth like a lamprey. Ergo, it was a fish.

Sallan and her team count the ways it isn’t a fish.

One: There are fossil lampreys from the era and they’re not this.

An illustration of Mazon Creek 300 million years ago, with a large shark, a salamander relative and two tiny Tully monsters.John Megahan

Two: The 2016 papers ignore how ironstone preserves fossils – one can see soft tissue but not internal structure.

Three: Evidence that the thing had complex eyes (those orbs at the end of the stalks) means nothing, because plenty of non-piscine invertebrates have complex eyes. Just ask an octopus.

Their examination of Tullimonstrum eyes led them to conclude that if anything, these creatures had primitive cup eyes that lack a lens.

“If it does have cup eyes, then it can’t be a vertebrate because all vertebrates either have more complex eyes than that, or [the monsters] secondarily lost them,” Sallan says. “But lots of other things have cup eyes, like primitive chordates, molluscs and certain types of worms.”

Nor has anybody found in the very many specimens any sign of structures that are believed to be universal in aquatic vertebrates, notably otic capsules, components of the ear that allow animals to balance, and a lateral line, a sensory structure that enables fish to orient themselves in space.

So what is the Tully monster? We don’t know.

Sallan and Giles coauthored the work with Robert Sansom of the University of Manchester, Penn postdoctoral researcher John Clarke, Zerina Johnason of the Natural History Museum London, Ivan Sansom of the University of Birmingham and Philippe Janvier of France’s Muséum National d’Histoire Naturelle.

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